Method and mold for producing multilayer absorbent tray



July 19, 1966 R. WELLS 3,261,740

METHOD AND MOLD FOR PRODUCING MULTILAYER ABSORBENT TRAY Filed Sept. 27, 1965 FIG, I

. INVENTOR Roger Wells KARL (U. :FLOQLS ATTORNEY United States Patent 3,261,740 METHOD AND MOLD FOR PRODUCING MULTILAYER ABSORBENT TRAY Roger Wells, Stamford, Conn., assignor to Diamond International Corporation, a corporation of Delaware Filed Sept 27, 1963, Ser. No. 312,197 20 Claims. (Cl. 162-123) This invention relates to methods and means for molding molded pulp articles. More particularly the invention relates to means and methods for the pulp molding of absorbent containers suitable for packaging foods.

Containers made of molded pulp are customarily used in most retail food markets today for packaging fresh meats, poultry, fish or other commodities from which some natural fluids such as blood and water are likely to exude. These containers are usually in the form of a shallow generally rectangular tray with a transparent covering sheet of cellophane or polyethylene wrapped around a tray and heat sealed to the bottom thereof. Such containers present an attractive display while providing adequate protection during the sale and temporary storage of food-stuff, particularly for naturally juicy fresh poultry and the like from which a very considerable free fluid drains.

In the case of a two-and-a-half pound chicken, the free fluid drain may be as much as 56 grams. A paperboard tray or a molded pulp tray used for packaging a 2 /2 lb. chicken would normally weigh between and grams. These trays must be very absorbent to have the capacity to pick up two or three times their weight of fluids draining from chicken or fresh meats. If the trays are made too absorbent, some rigidity may be lost after they become wet. Thus, if the entire body of the molded pulp tray were to be made of an unsized grade of pulp to render it inherently greatly absorbent, the tray would become soggy and weakened from the fluid and this would result in an unattractive package and possible leakage from the package, both of which must be avoided.

It has been proposed that certain wet strength resins could be incorporated into the molded pulp to prevent weakness caused by sogginess but this does not sufliciently overcome the above difiiculties. Furthermore, a significant disadvantage of this type of tray would be that it would exhibit a tendency to dehydrate the meat or poultry contained in the package; this also must be avoided. It has also been proposed in some cases to provide extra absorbency by placing a pad made of creped absorbent wadding with a glassine cover in the bottom of the tray so that the free juices run around the edges of the glassine and become absorbed by the wadding. These pads, however, are expensive and troublesome to insert into the tray.

A successful solution to the above problem has been accomplished by the production of a two-ply tray wherein the inside layer has a high absorptive capacity and the outside layer is hard-sized and non-absorbent. A tray of this nature can be provided with some sort of barrier to prevent direct contact between the food product and the absorbent tray which might .tend to dehydrate the product. This barrier may be a porous or perforated film and such a tray is shown in Patent #3,040,948 issued to Roger Wells. The inside layer of a tray of this type may be made of a very low density, coarse pulp which will felt together to make a pad with many voids, thus providing space for free liquid. The inner layer of this need not be so absorptive as to require a barrier film or coating between it and the product packaged.

The present invention relates to an improved process and apparatus for making a two-ply molded pulp tray wherein the inside layer of pulp is primarily restricted to the bottom of the tray and is made of fluify, highly ab- "ice sorbent material which need not have appreciable mechanical strength. This is accomplished by selectively depositing a particular type of absorbent material on the bottom forming surface of a pulp-mold in accordance with the present invention and then applying over the entire mold and deposited absorbent layer, a conventional pulp stock used in the manufacture of food trays.

It is therefore an object of the present invention to provide a new and improved method of forming two-ply molded pulp articles.

It is another object of the present invention to provide a new and improved method of forming pulp containers having absorbent inside layers.

It is another object of the present invention to provide means for forming a highly absorbent food container.

It is another object of the present invention to provide a simplified yet highly effective and inexpensive method of forming a food tray having a highly absorbent inside layer.

It is another object of the present invention. to provide a new and improved method and apparatus for molding improved pulp articles particularly suitable for packaging fresh poultry and other naturally wet or juicy foods.

It is another object of the present invention to provide a new process and mold which provides for the formation of essentially uniform layers of absorbent and nonabsorbent materials.

Other objects and the nature and advantages of the instant invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a view partly in section of a suction mold in accordance with the present invention;

FIG. 2 is a similar view showing the first deposition of stock;

FIG. 3 is a similar view showing the second deposition of stock; and

FIG. 4 is an isometric view in section of a molded pulp tray produced by the present invention.

The mold of the present invention, shown in FIG. 1, is provided with a backing plate 10 adapted to be connected to a suitable source of suction via a pipe 12. A main body portion 14 abuts the backing plate 10 and is contoured to produce an article in any desirable shape, generally a rectangular shallow tray. A first foraminous wire straining screen 16 is provided over the forming face of the body 14 by means of a peripheral retaining ring 18. Over only the bottom portion of the body 14 adjacent the area where it is desired to produce an absorbent layer in the molded pulp article is provided a second and generally coarser screen 20 intermediate the body 14 and the first screen 16. The second screen 20 may be retained in position merely by frictional engagement between the body 14 and the first screen 16, but preferably the second screen 20 is spot soldered to the body 14.

A third foraminous wire straining screen 19 may optionally be provided between the body 14 and the first screen 16 adjacent only that area where an essentially single layer is desired, and, as with screen 20, may be retained in some suitable manner such as merely by friction or by soldering. It is essential, however, that if third screen 19 is provided, there .must be a gap 21 between screens 19 and 20 of at least /2 inch adjacent an area 21. In lieu of a gap where screens 19 and 20 abut or where they are formed of a single screen, the screen adjacent area 21 must be filled with a substance to block off the screen so that it will not be possible to obtain lateral fiow from the bottom forming section to the side forming section of the mold. Suitable hole or screen blocking materials are plastics, resins, grease, putty, rubber, solder, etc.

The first straining screen 16 may be of the ordinary 'necessary and provides no advantage.

well known type for producing pulp molded articles. Such screens usually have a fine mesh of about 50 or 60 mesh. The intermediate screen 20, on the other hand, is preferably a coarser screen, being as coarse as 24 mesh, although a 30 mesh screen is preferable and adequate. The intermediate screen 20 may have as fine a mesh as the first screen 16, but if finer than 30 mesh, it has a tendency to plug up with pulp. Third screen 19 should not be coarser than 30 mesh and may be the same mesh as the first screen 16.

The main body portion 14 of the mold is perforated at areas of the forming face where the molding of both two-ply and essentially one-ply structure is desired. However, one type of perforation 22 is provided in areas where the molding of two-ply structure is desired Whereas perforations 24 are provided where only an essentially one-ply structure is desired. It is essential that the perforations 22 be greater in area than the perforations 24, so that the bottom of the mold has a significantly greater water draining capacity than the side of the mold. A general ratio of perforation open area 22 to perforation open area 24 is about 12 to 1. The range of effective surface area perfo ration forfthe perforations 22 has been found to lie generally between about 7.5 to about 45 percent of the area (on inch centers of 16 holes to the square inch, the diameters may vary between and inch) adjacent the section of the product wherein two plies are desired. The preferred size of holes 22 are 4.; inch in diameter on inch centers. These holes may be slightly smaller but must be large enough to carry away the water in the free pulp. The diameter of holes may be increased to li inch on inch centers, but this is not The preferred range of open area of holes 22 is about 12 to 30%.

The perforation 24, on the other hand, should comprise only about 0.2 to about 1.5 percent of the surface area (on A2 inch centers of 4 holes to the square inch, the diameters may vary between 0.030 and 0.070 inch) of the portion of the product wherein only one layer is desired. Here, the preferred range of open area is about 0.3 to 1.4%. Experimentation has indicated that when using an absorbent stock with a Canadian Standard Freeness of 700 for the absorbent layer and a covering stock -of Canadian Standard Freeness of 180 for the body portion of the product, the optimum number of holes 22 per square inch in the bottom is 16 holes at least of an inch in diameter whereas in the sides there should be four holes 24 per square inch, ,4 of an inch in diameter.

In accordance with the procedural aspects of the present invention, the mold 14 is first passed into a stock having a water draining capacity greater than the water draining capacity of holes 24 and vacuum is applied through pipe 12 while in such free stock. When vacuum is applied behind the mold the rate of build-up of the free stock is fast over the larger and more closely spaced holes 22 but restricted over the smaller holes 24 by their limited capacity to carry away the water. This provides an absorbent layer 26 which covers essentially only the bottom of the mold adjacent large holes 22 but which also extends as a thin, insignificant layer over holes 24. After the free stock has been deposited as absorbent layer 26 on the wire 16 adjacent the large hole 22, the mold 14 is withdrawn from the free stock and moved into a second vat of the conventional furnish with a stock having a water draining capacity less than the water draining capacity of holes 24. Vacuum is again applied and the conventional low freeness stock builds up across the entire mold as layer 28. The factor that limits the rate that this second stock builds up is the slow draining rate of the stock itself and not the size of the holes in the mold. The small holes 24 in the side of the mold having a water draining capacity greater than the stock itself will still carry away the water just as well as the more numerous and larger holes 22 in the bottom.

The absorbent inner layer 26 of the product should be inexpensive and bulky and have an absorptive weight ratio of over 7 parts by weight of water to 1 part by weight of dry pulp. A high bulk factor has two advantages: the high bulk increases the volume available for absorbency and also permits a higher drying rate to the conventional molded pulp tray into which it is formed.

Restricting the build-up of free stock to essentially a specific area of the mold in accordance with the present invention is accomplished by providing more and larger holes 22 in such specific area in comparison with the other areas where it is not necessary to have the absorbent layer and where the drainage holes 24 are of the minimum cross sectional area necessary to form the cover sheet of comparatively slow stock. The formation of the two ply tray wherein the inside layer is essentially restricted to the bottom area of the dish is dependent not only on the proper mold, but also upon the fact that the absorbent layer is made of very free stock which has a high drainage rate. During formation a large volume of water is rapidly pulled out of the stock as it is initially deposited on the wire. This stock can be something like unrefined sulphite, sulphate or other characteristically free chemical filler. It can also be made of fibers used for making cloth such as wool, hemp, jute or bagasse. The pulp stock from which absorbent layer 26 is formed should have a Canadian Standard Freeness of over 500. The second layer 28, on the other hand is made of conventional ground wood at a Canadian Standard Freeness of below 250 from which the water drains comparatively slowly.

The free pulp stock utilized for the absorbent layer deposition 26 should preferably be thin in consistency in order to obtain better control of the build-up. Such a reduced consistency should be in the neighborhood of to 7 of 1%. Inasmuch as the consistency of the regular stock used to cover the absorbent layer is in the neighborhood of from 1% to 1.3%, this more dilute consistency of the free, absorbent stock greatly reduces the build-up of any of such stock over the small holes 24. The lower consistency also improves the formation of layer 26 from the free stock.

In order to control the build-up of the free stock further and to restrict such build-up on the sides of the mold in the area of the small holes 24, it has also been found advantageous to use a lower vacuum than is normal as the mold is submerged in the free stock. This vacuum can be in the neighborhood of from 1 to 5 inches of mercury depending upon the desired thickness of the absorbent layer of free stock as compared with the normal vacuum of 15 to 20 inches of mercury which is utilized in the deposition of the second layer.

Example 1 A mold such as shown in FIG. 1 is utilized with holes 22 comprising 16 holes per square inch of "A of an inch in diameter and holes 24 comprising 4 holes per square inch of of an inch in diameter. Coarse screen 20 is 30 mesh and fine screen 16 is 50 mesh; no screen 19 is utilized. The mold is passed into a stock having a Canadian Standard Freeness of 700 and while therein vacuum of 3 inches of mercury is applied through pipe 12 to obtain absorbent deposit 26. The free stock has a consistensy of A of 1 percent. After deposition of absorbent layer 26, the mold is deposited in a second stock having a Canadian Standard Freeness of the stock consistency is 1 percent. A vacuum of 16 inches of mercury is applied through pipe .12 to form deposit 28. A product such as shown in FIG. 4 is the final result. This product is desirable in that the inner layer 26 absorbs up to 7 times its weight in liquid and yet the outer layer 28 remains strong and dry. 7

As a control a second mold is utilized which is identical in every respect to the mold described above except that no holes 24 are provided and only one screen v16 is provided (screen 20 as well as screen 19 is absent). A procedure identical to that above is carried out utilizing the control molds. In the product produced, the layer corresponding to layer 28 is found to thin out as it approaches edge of the mold and the sides of the tray are found to be insufficiently strong and to contain a high water content prior to drying. In addition, the drainage while producing the layer corresponding to layer 28 is slow and the screen '16 is found to plug up quickly.

Examples 2-10 The following chart, giving operating conditions and results, shows the satisfactory operating conditions lying within the scope of the present invention to produce desirable products.

free chemical wood pulp, wool, hemp, jute and bagasse Example 2 3 4 Canadian Standard Freeness of free pu p.

Canadian Standard Freeness of 2nd pulp.

Stock Consistency of Free Pulp.--"

Stock Consistency of 2nd Pulp '7 Vacuum in inches Hg during 1st 1 deposition. Vacuum in inches Hg during 2nd deposition.

Holes adjacent double layer Z64" A 251a diamediamediametcr on ter on ter on u t centers centers centers Holes adjacent single layer A 6 $132 A sf diamediamediameter on ter on centers centers. 1st screen" 60 mesh... 50 mesh.-. 2nd screen. 26 mesh.-- 30 mesh-.. 30 mesh... 3rd screen gap between 2nd and no screen mesh... mesh..-

3rd screens). used.

ter on 0 V /2 la" l Vi W centers. 50 mesh 30 mesh 24 mesh-.. 30 mesh.-- no screen used.

centers. 40 mesh. 26 mesh 40 mesh.

mesh... 24 mesh-.. 30 mesh...

no screen 30 mesh.

used.

It may be noted that the above examples set forth only the most desirable and practical limits, but that 3 such limits are not critical. The only cnitical features in producing a two-ply pulp article, wherein one ply covers essentially only a part of the mold and the other ply covers the whole mold, are that the water draining capacity of the first pulp must be greater than the water draining capacity of the smallest holes (in the mold area adjacent .to which significant deposits of the first pulp are not desired) and the water draining capacity of the second pulp must be less than the water draining capacity of the smallest holes in the mold, the second deposit thereby covering the entire mold.

It will be obvious to those skilled in the art that various changes may be made without departing from the scope of the invention and therefore the invention is no limited to what is shown in the drawings and described in the specification but only as indicated in the appended claims.

What is claimed is:

1. A method of forming a multi-ply pulp molded article comprising: passing a selectively perforated mold having small holes at one part and large holes at another part through a pulp stock having a water draining capacity greater than the water draining capacity of the small holes in said mold, applying a differential pressure to said mold in said stock to effect selective depositon of pulp from said stock on said mold at said other part, passing said mold with said selective deposit of pulp through a second pulp stock having a water draining capacity less than the water draining capacity of .the small holes in said mold, and applying a differential pressure to said mold in said second stock to effect deposition over the entire surface of said mold and first deposit.

2. The method of claim 1 wherein said other part is the bottom forming portion of the mold and said selective deposit covers essentially that bottom forming portion.

3. The method of claim 1 wherein the differential pressure is applied by applying vacuum through the mold.

4. The method of claim 3 wherein the first vacuum applying step utilizes a weak vacuum.

5. The method of claim 4 wherein said weak vacuum and wherein the pulp of said second stock is substantially 5 ground wood.

8. The method of claim 1 wherein the consistency of the first stock lies between about and A of 1% and the consistency of said second stock lies between about 1% and 1.3%.

9. The method of claim 2 wherein the solid portion of the bottom of said perforated mold comprises from about 92.5% to 55% of the bottom surface area of said mold and wherein the solid portion of the sides of said perforated mold comprises from about 99.8% to about 98.5% of the side surface area of said mold.

10. The method of claim 9 wherein the first stock has a Canadian Standard Freeness of between about 550 and 800 and said second stock has a Canadian Standard Freenes of between about and 220 and wherein the bottom forming portion of said mold has an open area lying between about 12% and 30% of the bottom area and the side of said mold has an open area lying betwen about 0.3% and 1.4% of the side area.

11. The method of claim 9 wherein the first stock has a Canadian Standard Freeness of about 700 and the bottom forming portion of said mold has about 16 holes per square inch each having a diameter of about of an inch and wherein said second stock has a Canadian Standard Freeness of about and the side of said mold has about 4 holes per square inch each having a diameter of about of an inch.

12. A mold for forming pulp articles comprising a body portion shaped to the desired configuration of the article to be produced, said body defining a first portion and a second portion, said body having walls defining perforations extending therethrough from one side of the body to the other side thereof, the perforations in said first portion comprising from about 7.5 to 45% of the surface area of said bottom, and the perforations in said second portion comprising 0.2 to 1.5% of the surface area of said side portion.

13. A mold in accordance with claim 12 further comprising a first screen extending along the outer surface of said body.

14. A mold in accordance with claim 13 further comprising a second screen disposed between said body and said first screen over only the first portion of said body.

15. A mold in accordance with claim 14 wherein said first screen has a fine mesh and said second screen has a coarse mesh relative to each other.

16. A mold in accordance with claim 15 wherein said fine mesh screen has a mesh of between about 50 and 60 and wherein said coarse mesh screen has a mesh of between about 24 and 30.

17. A mold in accordance with claim 12 wherein said perforations are cylindrical, and about 16 perforations per square inch of about of an inch in diameter are present in said first portion, and about 4 perforations per square inch of about inch in diameter are present in said second portion.

18. A mold in accordance with claim 14 further comprising a third screen disposed between said body and said first screen over only said second portion of said body, said third screen and said second screen having a screen gap betwen their approaching edges sufiicient to inhibit lateral flow between said second and third screens.

19. A mold in accordance withi claim 18 wherein said gap comprises a solid filler material.

20. A mold in accordance with claim 18 wherein said first portion corresponds to the bottom of said mold and said second portion corresponds to the side of said mold.

References Cited by the Examiner UNITED STATES PATENTS 2,079,667 5/1937 Swift 162228 2,995,187 8/1961 Wells 162-390 3,011,546 12/1961 Peppler 16239O FOREIGN PATENTS 660,115 10/ 1951 Great Britain.

DONALL H. SYLVESTER, Primary Examiner.

JOHN H. NEWSOME, Assistant Examiner. 

1. A METHOD OF FORMING A MULTI-PLY PULP MOLDED ARTICLE COMPRISING: PASSING A SELECTIVELY PERFORATED MOLD HAVING SMALL HOLES AT ONE PART AND LARGE HOLES AT ANOTHER PART THROUGH A PULP STOCK HAVING A WATER DRAINING CAPACITY GREATER THAN THE WATER DRAINING CAPACITY OF THE SMALL HOLES IN SAID MOLD, APPLYING A DIFFERENTIAL PRESSURE TO SAID MOLD IN SAID STOCK TO EFFECT SELECTIVE DEPOSITION OF PULP FROM SAID STOCK ON SAID MOLD AT SAID OTHER PART, PASSING SAID MOLD WITH SAID SELECTIVE DEPOSIT OF PULP THROUGH A SECOND PULP STOCK HAVING A WATER DRAINING CAPACITY LESS THAN THE WATER DRAINING CAPACITY OF THE SMALL HOLES IN SAID MOLD, AND APPLYING A DIFFERENTIAL PRESSURE TO SAID MOLD IN SAID SECOND STOCK TO EFFECT DEPOSITION OVER THE ENTIRE SURFACE OF SAID MOLD AND FIRST DEPOSIT.
 12. A MOLD FOR FORMING PULP ARTICLES COMPRISING A BODY PORTION SHAPED TO THE DESIRED CONFIGURATION OF THE ARTICLE TO BE PRODUCED, SAID BODY DEFINING A FIRST PORTION AND A SECOND PORTION, SAID BODY HAVING WALLS DEFINING PERFORATIONS EXTENDING THERETHROUGH FROM ONE SIDE OF THE BODY TO THE OUTER SIDE THEREOF, THE PERFORATIONS IN SAID FIRST PORTION COMPRISING FROM ABOUT 7.5 TO 45% OF THE SURFACE AREA OF SAID BOTTOM, AND THE PERFORATIONS IN SAID SECOND PORTION COMPRISING 0.2 TO 1.5% OF THE SURFACE AREA OF SAID SIDE PORTION. 